A reciprocating engine to be employed in a motor vehicle, a motorcycle, an agricultural machine, a marine vessel or the like requires a crankshaft to extract power by converting reciprocating motions of pistons to rotational motion. There are two types of crankshafts: the type manufactured by die forging and the type manufactured by casting. Especially when high strength and high stiffness are required, die forged crankshafts (which will hereinafter be referred to as “forged crankshafts”) are often employed.
FIGS. 1A and 1B are schematic diagrams showing an example of a shape of a commonly used crankshaft. FIG. 1A is an overall view, and FIG. 1B is a sectional view along the line IB-IB. In order to facilitate understanding of the shape of the crankshaft, FIG. 1B shows only a crank arm A7, a counterweight W7 integrated with the crank arm A7, a pin P4 and a journal J4 connected to the crank arm A7, which are extracted from the crankshaft.
The crankshaft 11 shown in FIGS. 1A and 1B is a four-cylinder eight-counterweight crankshaft to be mounted in a four-cylinder engine. The crankshaft 11 includes five journals J1 to J5, four pins P1 to P4, a front part Fr, a flange Fl, and eight crank arms (hereinafter referred to simply as “arms”) A1 to A8. The eight arms A1 to A8 connect the journals J1 to J5 respectively to the pins P1 to P4. The eight arms (all of the arms) A1 to A8 have counterweights (hereinafter referred to simply as “weights”) W1 to W8, which are integrally formed with the arms A1 to A8, respectively. The front part Fr is located at a front end of the crankshaft 11, and the flange Fl is located at a rear end of the crank shaft 1, the front end and the rear end being ends in a direction along the axis of the crankshaft 11. The front part Fr is connected to the front first journal J1, and the flange Fl is connected to the rearmost fifth journal J5.
In the following paragraphs, when the journals J1 to J5, the pins P1 to P4, the arms A1 to A8, and the weights W1 to W8 are each collectively referred to, a reference character “J” is used for the journals, a reference character “P” for the pins, a reference character “A” for the arms, and a reference character “W” for the weights. An arm A and a weight W integrated therewith are referred to collectively as a “web”.
As shown in FIG. 1B, the width Bw of the weights W is greater than the width Ba of the arms A. Accordingly, each of the weights W bulges greatly from an arm center plane (a plane including the axis of the pin P and the axis of the journal J).
A forged crankshaft having such a shape is generally produced by using a billet as a starting material. A section of the billet in a direction perpendicular to the longitudinal direction thereof, that is, a cross section of the billet is circular or square, and the cross-sectional area is constant throughout the length. In the following paragraphs, a section of a crankshaft in a direction perpendicular to the axis of the crankshaft is referred to as a “cross section”, and a section of the crankshaft in a direction parallel to the axis of the crankshaft is referred to as a “longitudinal section”. The area of the cross section is referred to simply as a “sectional area”. A method for producing a forged crankshaft includes a preforming step, a die forging step, and a trimming step that are to be executed in this order. After the trimming step, a coining step may be executed if needed. Typically, the preforming step includes a rolling step and a bending step, and the die forging step includes a rough forging step and a finish forging step.
FIGS. 2A to 2F are schematic diagrams showing a conventional method for producing a common forged crankshaft. FIG. 2A shows a billet, FIG. 2B shows a rolled blank, FIG. 2C shows a bent blank, FIG. 2D shows a rough forged blank, FIG. 2E shows a finish forged blank, and FIG. 2F shows a forged crankshaft. FIGS. 2A to 2F show a method for producing a crankshaft having the configuration shown in FIGS. 1A and 1B.
In the production method shown in FIGS. 2A to 2F, a forged crankshaft 11 is produced as follows. First, a billet 12 with a specified length as shown in FIG. 2A is heated in a heating furnace, and in a preforming step, the heated billet is rolled and subsequently bent. In the rolling, the billet 12 is rolled and reduced, for example, by grooved rolls. This is to distribute the volume of the billet 12 in the axial direction, and thereby, a rolled blank 13, which is an in-process material, is obtained (see FIG. 2B). Next, in the bending, the rolled blank 13 is partly pressed and reduced from a direction perpendicular to the axial direction. This is to distribute the volume of the rolled blank 13, and thereby, a bent blank 14, which is a next in-process material, is obtained (see FIG. 2C).
Next, in a rough forging step, the bent blank 14 is forged by a pair of an upper die and a lower die, and thereby, a rough forged blank 15 is obtained (see FIG. 2D). The rough forged blank 15 is roughly in the shape of the crankshaft (final product). In the finish forging step, the rough forged blank 15 is forged by a pair of an upper die and a lower die, and thereby, a finish forged blank 16 is obtained (see FIG. 2E). The finish forged blank 16 has a shape in agreement with the shape of the final product, that is, the crankshaft. During the rough forging and the finish forging, excess material flows out through a space between the mutually facing parting faces of the dies, which results in formation of flash B. Accordingly, the rough forged blank 15 and the finish forged blank 16 have great flash B on the periphery.
In a trimming step, for example, while the finish forged blank 16 is nipped and held by a pair of dies, the finish forged blank 16 is punched by a cutting die. Thereby the flash B is removed from the finish forged blank 16, and a forged blank with no flash is obtained. The forged blank with no flash has substantially the same shape as the forged crankshaft 11 shown in FIG. 2F.
In a coining step, main parts of the forged blank with no flash are slightly pressed by dies from above and below so that the forged blank with no flash can have the exact size and shape of the final product. The main parts of the forged blank with no flash are, for example, shaft parts such as the journals J, the pins P, the front part Fr, the flange Fl and the like, and further, the arms A and the weights W. In this way, the forged crankshaft 11 is produced.
The production method shown in FIGS. 2A to 2F is applicable not only to production of a four-cylinder eight-counterweight crankshaft as shown in FIGS. 1A and 1B but also to production of any other crankshaft. For example, the production method is applicable to a four-cylinder four-counterweight crankshaft.
In a four-cylinder four-counterweight crankshaft, only some of the eight arms A1 to A8 incorporate a weight W. For example, the front first arm A1, the rearmost eighth arm A8 and the central two arms (the fourth arm A4 and the fifth arm A5) incorporate a weight W. The other arms, namely the second, the third, the sixth and the seventh arms (A2, A3, A6 and A7) do not have a weight, and these arms are like oval-shaped.
Other crankshafts, for example, crankshafts to be mounted in three-cylinder engines, in-line six-cylinder engines, V-type six-cylinder engines, eight-cylinder engines and others can be produced by the same production method. It is noted that, when adjustment of the placement angles of the pins is necessary, a twisting step is added after the trimming step.
The main purpose of the preforming step is distributing the volume of the billet, and therefore, the blank obtained thereby is hardly in the form of the forged crankshaft. By distributing the volume of the billet in the preforming step, it is possible to decrease the outflow of material and accordingly to decrease the formation of flash in the next die forging step, thereby improving the material yield rate. The material yield rate means the rate (percentage) of the volume of the forged crankshaft (final product) to the volume of the billet.
For example, Japanese Patent Application Publication No. 2001-105087 (Patent Literature 1), Japanese Patent Application Publication No. H2-255240 (Patent Literature 2) and Japanese Patent Application Publication No. H10-029032 (Patent Literature 3) disclose techniques relating to production of a forged crankshaft. Patent Literature 1 teaches a preforming step using a pair of an upper die and a lower die. During pressing of a rod-like workpiece by use of an upper die and a lower die in the preforming step, while a part of the workpiece is elongated, another part connecting thereto is offset from the axis. In the preforming step disclosed in Patent Literature 1, rolling and bending are performed at the same time, which allows a decrease in investment for facilities.
According to Patent Literature 2, in a preforming step, a four-pass high-speed rolling, rather than a conventional two-pass rolling, is performed. A rolled blank obtained by the preforming step have sectional areas that are congruent with the sectional area distribution among weights, arms and journals of the forged crankshaft (final product). According to Patent Literature 2, this improves the material yield rate.
Patent Literature 3 suggests that pressing direction (pressing direction) in a die-forging step should be perpendicular to a bulging direction of weights. Thereby, in the die-forging step, the degree of filling of material in the weights greatly bulging from the arm center plane can be improved. In the method disclosed in Patent Literature 3, the parting faces of the upper die and the lower die are located at the vertexes of the bulging weights, and accordingly, excess material flows out through the space between the upper die and the lower die and forms into flash.